1. Field of the Invention
The present invention relates to a die-bonding machine used in fabricating a semiconductor device.
2. Description of Related Art
Generally, a semiconductor device is fabricated by sequentially assembling a semiconductor pellet (simply called a "pellet" hereinafter) on a pellet mounting section (called a "land" hereinafter) of a supporting member such as a lead frame or a BGA (ball grid array) board, by means of a die-bonding machine.
Now, a die-bonding system widely used in the prior art will be described with reference to the drawings. FIG. 1 is a diagrammatic partial plan view illustrating the relation between a semiconductor wafer and a mechanism for feeding a supporting member such as a lead frame, and FIG. 2 is a diagrammatic side sectional view of the mechanism shown in FIG. 1.
The supporting member 1, such as the lead frame, having a land on which the pellet is to be mounted and on which a bonding agent such as a solder or a silver (Ag) paste was placed in a preceding step, is loaded on a rail 2, and intermittently fed and then stopped in a left-to-right direction in FIG. 1 (namely, in an X direction) by a feeding mechanism (not shown) so that the land is placed at a predetermined bonding position BP.
On the other hand, a number of pellets 3 obtained by dicing a wafer are adhered on a sheet (not shown) to maintain a geometrical arrangement before the wafer is divided into the pellets, and the sheet is fixed to a wafer ring 5. The wafer ring 5 is fitted into and held by a ring holder 6, which is supported by a supporting and displacing mechanism (not shown). This supporting and displacing mechanism can freely displace the ring holder 6 (and hence the pellets 3 adhered on the sheet) in the X direction and in a Y direction perpendicular to the X direction, and also can freely rotate the ring holder 6 (namely, it can displace in an angular direction (a .theta. direction)), so that a designated pellet is sequentially placed in a fixed pick-up position PP one by one. This fixed pick-up position PP is determined to be as near to the bonding position BP as possible, in the Y direction from the bonding position BP.
Furthermore, a camera 7 is located and fixed directly above the fixed pick-up position PP, and an image of the pellet located on the fixed pick-up position PP, is photographed by the camera 7 and processed by a processing system (not shown) to ascertain whether or not a bad mark exists, and whether or not an appearance abnormality such as a crack or a chipping exists, and also to confirm that the pellet is positioned in place on the fixed pick-up position PP. If the pellet photographed by the camera 7 was defective, the ring holder 6 is displaced by the supporting and displacing mechanism so that a next pellet is moved to the fixed pick-up position PP. If the next pellet is non-defective, the ring holder 6 is finely displaced by the supporting and displacing mechanism so that the next pellet is positioned precisely on the fixed pick-up position PP. A push-up pin 8 is provided under the ring holder 6, in substantial alignment with the camera 7 in a vertical line.
The die-bonding machine also includes a pick-up collet having a vacuum nozzle 4 provided at a lower tip end. The pick-up collet can be moved so that the vacuum nozzle 4 can be displaced in a vertical direction (namely, in a Z direction) and also in the Y direction, shown by a dotted line in FIG. 2.
Thus, when the pellet located on the fixed pick-up position PP is non-defective, the ring holder 6 is finely displaced by the supporting and displacing mechanism so that the pellet is positioned precisely on the fixed pick-up position PP. The vacuum nozzle 4 is displaced to suction up the pellet, and on the other hand, the push-up pin 8 is elevated to push up the pellet and to peel off the pellet from the adhering sheet fixed by the wafer ring 5. After the pellet is suctioned and held by the vacuum nozzle 4, the vacuum nozzle 4 is lifted up, and moved in the Y direction to a position directly above the bonding position BP, and then, is lowered so that the pellet sucked and held by the vacuum nozzle 4 is placed on the land of the supporting member 1. Thereafter, the vacuum nozzle 4 is released and then lifted up. Thus, the non-defective pellet is placed on the land of the supporting member 1.
In the prior art die-bonding machine constructed as mentioned above, the bonding position BP and the fixed pick-up position PP are located as near to each other as possible, so that the time for carrying the pellet 3 is shortened, with the result that a high speed die-bonding operation can be realized. To the contrary, a moving range of the ring holder 6 becomes a double of the diameter of the wafer, so that the machine becomes large in size. Recently, since the wafer diameter has become 8 inches or 12 inches, this problem has become even worse.
Under this circumstance, Japanese Patent Application Pre-examination Publication No. JP-A-01-152634, (an English abstract of which is available and the content of the English abstract is incorporated by reference in its entirety into this application) proposes a die-bonding machine, which is so configured to imaginarily divide a number of pellets held by the ring holder into four quadrants by two orthogonal straight lines, and to limit the moving range of the ring holder in the X and Y directions to the degree that the pellets included in only one quadrant can be placed on the fixed pick-up position, and on the other hand to rotate the ring holder by 90 degrees at each time the pick-up operation for one quadrant is completed, so that the pick-up operation is carried out quadrant by quadrant. In this proposed die-bonding machine, the moving range of the ring holder in the X and Y directions can be reduced to one and a half times the diameter of the wafer. Therefore, even the time for rotating the ring holder becomes wasteful, since all the other operations can be sped up, the die-bonding operation can be sped up in total.
However, since the direction of the pellets changes by the rotation of the ring holder, it is necessary to change the direction of the picked-up pellet in the course of carrying the picked-up pellet to the supporting member, excepting for a special pellet which is formed in symmetry. Therefore, it is required that the vacuum suction nozzle of the pellet picking-up and carrying mechanism is constructed to be rotatable so as to adjust the direction of the picked-up pellet in the course of carrying the picked-up pellet, or alternatively, a rotatable relay stage is provided and the pellet picking-up and carrying mechanism is controlled to carry the picked-up pellet to the rotatable relay stage so that the pellet put on the rotatable relay stage is rotated together with the rotatable relay stage to adjust the direction of the pellet and the direction-adjusted pellet is picked up by another vacuum nozzle and carried and located on the land of the supporting member by the second vacuum nozzle.
As seen from the above, in this prior art die-bonding machine, since the direction of the pellet is different from one pellet to another when the pellet is picked up, it is necessary to adjust the direction of the pellet in the course of carrying the picked-up pellet. This direction adjusting operation is different from one pellet to another, and therefore, control software becomes very complicated.
Furthermore, Japanese Patent Application Pre-examination Publication No. JP-A-04-111330, (an English abstract of which is available and the content of the English abstract is incorporated by reference in its entirety into this application), proposes another die-bonding machine which does not have the fixed pick-up position. In this second prior art die-bonding machine, the ring holder is rotatable about its center axis (namely, displaceable in the .theta. direction), but the position of the center axis is fixed both in the X direction and in the Y direction. The rotating operation of the ring holder is only for aligning the direction of each pellet, but when each pellet is picked up, the ring holder is not rotated. On the other hand, the camera is displaceable both in the X direction and in the Y direction, as to sequentially ascertain the defective/non-defective of the pellet and the position of the pellet. The vacuum suction nozzle of the pellet picking-up and carrying mechanism is movable all in the X direction, in the Y direction and in the Z direction to move to the pellet which was ascertained by the camera, to pick up the same pellet, to move in the X direction and in the Y direction so as to locate the picked-up pellet on a predetermined location. This Japanese patent publication has no description concerning the push-up pin, but if the push-up pin is provided, the push-up pin is required to move both in the X direction and in the Y direction to follow the displacement of the camera.
In this second prior art die-bonding machine, since the ring holder is fixed in the X direction and in the Y direction, and the moving range of the pellet suction nozzle of the pellet picking-up and carrying mechanism is substantially within the ring holder, the machine becomes smaller in size. However, since the pellet picking-up and carrying mechanism has to go to a remote position for picking up the pellet, the die-bonding operation becomes slow. Therefore, it is supposed that the second prior art die-bonding machine can be applied in only the case that a high speed die-bonding operation is not required because a step before the die-bonding step and a step after the die-bonding step in an in-line production line need a substantial time. For example, since the silver paste needs a long time for hardening, when a reflow oven becomes long for a high speed feeding, the second prior art die-bonding machine can be applied. Alternatively, when a wire bonding needs a long dime because of a large number of pins, it is sufficient if the die-bonding is carried out at a speed comparable to that of the wiring bonding, the second prior art die-bonding machine can be applied.
In any case, since the second prior art die-bonding machine remarkably lowers the die-bonding speed, the second prior art die-bonding machine cannot be applied generally.